High purity (99.6% purity) titanium (Ti) plate (30 mm×30 mm×1 mm) was used as the substrate, the surface of which was firstly polished with 600 grit-sand papers and cleaned with deionized water. Then the Ti plate was etched in 10wt% oxalic acid at 98℃ for 2 h, followed by thoroughly washing with deionized water before use. The synthesis solution was prepared by dissolving 10% SnCl₄·5H₂O and 1% SbCl₃ in absolute alcohol, subsequently adjusted the pH of solution to 1.

The dip-coating method procedure was as follows: the pretreated titanium plate was brushed with the precursor solution, followed by drying at 120℃ for 15 min, and then it was calcinated in a muffle oven at 450℃ for 15 min. This procedure was repeated for 10 times. For the last time, the electrode was annealed at 450℃ for 1 h to induce phase transformation.

The microwave method (Multiwave 3000, Anton Paar) procedure was as follows: The pretreated titanium plate was fixed in a Teflon lined microwave digestion vessel containing the synthesis solution and then the vessel was sealed. The apparatus parameters were set as follows: 400 W in power, pressure of 4 MPa, heating up time of 15 min, retention time of 90 min and cooling time of 15 min. The as-prepared electrode was then washed thoroughly with deionized water and dried in air. The calcination process (CMF1100, Hefei Kejin Co. Ltd, China) was also carried out subsequently at 450℃ for 1 h.

The morphology of the metal oxide coating film was examined by SEM (JEOL, JSM-6700F). The crystal structure of the as-prepared electrodes was characterized by a Rigaku D/MAX-2400X X-Ray Diffractometer with Cu Kα radiation.

The electrochemical characteristic measurement of the prepared electrodes was performed in a three-electrode cell at room temperature, with the as-prepared electrode serve as the working electrodes, Pt plate as the counter electrode and Ag/AgCl/saturated KCl electrode as the reference electrode. The electrochemical measurement was conducted on the electrochemical working system (LK3200A, Lanlike, China) at room temperature. The linear scanning voltammetry was carried out at a sweep speed of 20 mV/s in the range of 0 V to and 2.5 V vs Ag/AgCl/saturated KCl electrode, with 20 g/L Na₂SO₄ solution as supporting electrolyte. For the accelerated life test, the current density was maintained at 100 mA/cm² in 0.5 mol/L Na₂SO₄ solution supporting electrolyte. The electrode was defined to be inactivated when the cell voltage reaches 10.0 V here.

All the degradation experiments were carried out with Acid Red G (ARG) as the simulated organic pollutant. The as-prepared electrode was served as the anode and the cathode was titanium sheet, with a distance between the two electrodes of 1.5 cm. The original ARG concentration was 50 mg/L and 0.1 mol/L Na₂SO₄was added to the aqueous solution as the supporting electrolyte. The experiments were carried out at room temperature (22±1)℃. Water samples were withdrawn from the solution every 20 min during the process for HPLC and TOC analyses. Total organic carbon (TOC) of the water sample was determined by a Shimadzu TOC 5000A apparatus. Waters HPLC system (515 HPLC pump, 2489 UV/visible detector and 717 plus autosampler) was used to identify the organic compounds with an ACE 5 C18 column (φ250 mm×4.6 mm). The composition of the mobile phase was 80% acetonitrile and 20% acetic acid (20 mL acetic acid in 1000 mL deionized water). The flow rate was 0.8 mL/min and the UV-Vis detector was set at 503 nm. The retention time of the probe in this condition was 2.8 min. The color removal efficiency was calculated from the probe peak area.

The morphology of the electrodes prepared by dip-coating method and microwave deposition method was investigated by SEM, respectively. Figure 1 (a) presents a typical mud-crack appearance of the Ti/Sb-SnO₂ electrode fabricated with dip-coating method. The cracks may allow the treated solution diffusing towards the titanium base, leading to the occurring of electrochemical reactions at the interface of the titanium base and the out-layer. For instance, oxygen radicals generated from the electrochemical reactions could oxidize the titanium base to insulated titanium dioxide thereby decreasing the lifetime of the electrode. Moreover, oxygen can impact the Sb-SnO₂ layer to split away from the titanium base, making the electrode unstable. While, Fig. 1 (b) shows the less smooth but compact surface of the electrode prepared by microwave approach. Some particles with particle size smaller than 5 μm were also observed to adhere onto the electrode surface. It can be obviously seen that there is much less cracks formed at the electrode surface compared to the dip-coating produced electrode. More compact surface can decrease the possibility of electrolyte diffusion into the titanium base; consequently, improve the stability of the electrode.

Fig. 1

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	Fig. 1 SEM images of the Sb-SnO2 electrodes(a) Dip-coating method; (b) Microwave method

XRD patterns of the electrodes prepared by both microwave and dip-coating method are shown in Fig. 2. The XRD pattern of the dip-coating fabricated electrode shows the typical diffraction peaks of titanium at 38°, 40°, 54° and 71°, respectively. Comparing to that, the microwave fabricated electrode shows weaker diffraction peaks, especially at the reflection peak at 40°. This phenomenon indicates that the titanium base is well covered by the metal oxide crystal formed on its surface, which is in accordance with the SEM analysis displayed above. The peaks at 26.51°, 33.80° and 51.85° are the characteristic diffraction peaks belonging to crystalline SnO₂.It is noticed that all the peaks, namely 26.61°(110), 33.89°(101) and 51.78°(211) (ICSD39173), shift towards higher angles from the standard crystalline SnO₂ diffraction peak, which is probably due to the solid solution formed between Sb and SnO₂. The ionic radius of Sb⁵⁺ and Sn⁴⁺ are 0.060 nm and 0.069 nm, respectively. The difference between them is 13%, which is less than the threshold value of 15% proposed by Hume-Rothery principle^{[ 6]}. Hence, it is likely to form solid solution between Sb and SnO₂. Considering the intensity of the Sb-SnO₂ diffraction peaks, there is no evident difference between the two types of electrodes. Nevertheless, the microwave produced electrode shows the sharper peak comparing with the dip-coating one, indicating that the higher crystallization degree is achieved by the microwave approach. According to Scherrer equation^{[ 17]}, the average crystal size of Sb-SnO₂ formed by the microwave method is 32.8 nm, while it is 27.5 nm for the dip-coating method one. In the dip-coating process, the repeated calcination steps make the metal oxide formed on the electrode surface disperse uniformly. But in the microwave process, one time calcination make the metal oxide disperse not so fully as the former one, consequently a larger crystal size is get .

Fig. 2

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	Fig. 2 XRD patterns of the Sb-SnO2 electrodes

The accelerated life tests were conducted to evaluate the electrochemical lifetime of the electrodes prepared by dip-coating and microwave method, respectively (Fig. 3). As we set 10 V ( vs Ag/AgCl) as the electrode failure criteria, the service life was about 95 min for the microwave fabricated electrode while it was only 25 min for the dip-coating one. It is well known that under the same circumstances, the electrode life depends on the characteristics of the electrode, specifically, the amount of metal oxide deposited on the electrode surface and the compactness of the electrode surface. In the regard of Sb/SnO₂ oxide loading, the dip-coating type of electrode was 1.1 mg/cm²while the microwave type of electrode was 3.2 mg/cm², suggesting the better effect of microwave fabrication. In the real application of dip-coating fabricated electrode, the electrolyte may diffuse through the cracks towards the titanium base, consequently cause the oxygen evolution reaction to occur. Oxygen would result in the formation of insulated TiO₂ layer between the titanium base and the out-layer (Sb-SnO₂), thus the deactivation of the electrode in a short time was achieved. In the case of microwave type of electrode, the more compact surface lead to a stronger resistance to the electrolyte diffusion into the titanium base and a decreased probability of oxygen evolution, thus a better stability and a longer lifetime are expected.

Fig. 3

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	Fig. 3 Accelerated life test of the Sb-SnO2 electrodes

As we know, the oxygen evolution reaction at the anode is the predominant side reaction in the electrochemical process. Figure 4 shows the linear sweep voltammetry of the electrodes prepared by dip-coating and microwave method, respectively. It can be clearly seen from Fig. 4 that the over potential of oxygen evolution at microwave fabricated electrode is 1.70 V (Ag/AgCl as the reference electrode), but that for dip-coating fabricated electrode is 1.66 V (Ag/AgCl as the reference electrode), indicating a declined possibility of oxygen evolution. The oxygen evolution mechanism is elucidated in Equations (1-3). First, water is decomposed to generate ·OH radicals at the SnO₂ electrode surface in the presence of electric field (Equation 1). Then the generated ·OH radicals are physically adsorbed onto the electrode surface to oxidize the surrounding organic compounds R (Equation 2). Meanwhile, the generated ·OH radicals collide with each other to produce oxygen (Equation 3)^{[ 18, 19]}. According to the XRD results analyzed above, the size of the crystal grain prepared by microwave method is greater than that of the dip-coating method. As a result, the probability of collision between ·OH radicals at the microwave fabricated electrode is less than the latter one in the view of statistics. Further, the oxygen evolution probability of the former electrode is less than the latter one, which is reflected in the linear sweep curve that the over potential of oxygen evolution at microwave electrode is higher than the dip-coating electrode.

SnO₂ + H₂O → SnO₂(·OH) + H⁺ +e^- (1)

·OH + R →·RO +[H] (2)

·OH +·OH → H₂O + 1/2 O₂↑ (3)

Fig. 4

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	Fig. 4 Linear sweep voltammetry of the Sb-SnO2 electrodes

To evaluate the electrocatalytical property of the electrodes prepared by two different approaches, electrochemical degradation of organic pollutants experiment was carried out with ARG chosen as the organic probe. The decoloration and degradation efficiency of ARG at dip-coating and microwave Sb-doped SnO₂electrodes are shown in Fig.5. It can be clearly observed from Fig. 5 (a) that the microwave electrode exhibits a slightly better performance in the decoloration effect of ARG than the dip-coating electrode does, with removal efficiency of 85.41% and 84.64% after treatment for 2 h, respectively. The TOC removal process shows a similar result with the color removal process at two different electrodes, with removal efficiency of 7.82% and 5.05% for microwave method and dip-coating method after 2 h treatment, respectively. This result is in consistent with the over potential of oxygen evolution measured in linear sweep voltammetry, that is to say, higher over potential results in more·OH radical accumulated at the microwave-fabricated electrode surface, compared with the dip-coating electrode. Consequently, a higher decoloration and degradation rate on microwave electrode is achieved. Though the enhancement is not obvious and encouraging, the microwave approach provides an alternative to fabricate Ti/Sb-SnO₂ electrode with less troublesome procedure and competitive result than the conventional dip-coating method.

Fig. 5

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	Fig. 5 Electrochemical treatment of ARG wastewater of the Sb-SnO2 electrodes(a) ARG removal; (b) TOC removal

High quality Sb doped SnO₂ electrode was successfully prepared on the Ti substrate by microwave method and dip-coating method, and the comparison in electrochemical performance between them was also discussed. The SEM images of the microwave-electrode reveal a more compact surface morphology than the dip-coating one, and a better and larger crystal of SnO₂ compared with the dip-coating electrode is calculated from the XRD pattern. The service life of the microwave electrode is measured to be 95 min while it is 25 min for the dip-coating electrode. The microwave electrode also demonstrates a higher oxygen evolution potential of 1.70 V than the dip-coating electrode (1.66 V). During the electrochemical degradation process, the microwave electrode exhibited a better performance both in the color and TOC removal efficiency. Furthermore, compared with the dip-coating method, the microwave method consumed less time and operation procedure which was energy efficient in modern society.